1. Field of the Invention
The present invention relates to a magneto-optical recording medium, and more particularly, to a magneto-optical recording medium having a reproducing layer of superlattice multilayer.
2. Background of the Related Art
As demands on high density information recording/reproduction have been increased recently, a high density recording using a laser beam has been paid much attention. Particularly, the magneto-optical disk is an attractive recording medium because it allows repetitive recording and erasure of information, and can easily implement the high density recording. There have been active researches on the magneto-optical disk.
Recording information by forming a magnetic domain in a perpendicular magnetic thin film using a laser beam and a magnetic field, and reproducing the information using a magneto-optical effect, the magneto-optical disk widely uses RE-TM(Rare Earth-Transition Metal) alloy groups as a recording layer material. The transition metal elements are Fe, Co, and etc., which are ferromagnetic, and the rare earth elements are Tb, Dy, Gd, Sm, and Ho, and etc. One of the most important objects of the magneto-optic disk is to record more information within a unit area and to reproduce the recorded information without any error. The strategy of laser-pumped magnetic field modulation used for recording more information within the unit area has facilitated formation of a small, but stable magnetic domains by applying laser pulses and an external magnetic fields simultaneously. Though the high density recording is possible by forming the smaller magnetic domain than the size of a recording laser beam in the recording, a new reproduction technology is required when the magnetic domain has a size substantially smaller than a size of the reproducing laser beam. Because the reproduction error increases with increasing noise and decreasing carrier-to-noise ratio(CNR) due to the infiltrated signal from adjacent magnetic domains when the recorded domains were much smaller than those of the reproducing laser beam.
Therefore, special reproduction methods are required for solving the problems.
The first method is MSR(Magnetically induced Super Resolution) in which an aperture of a reproduction layer is opened in the high temperature area of center portion of the reproducing laser beam having a Gaussian temperature distribution in order to duplicate a signal from the recording layer. In a CAD(Center Aperture Detection) MSR, a direction of magnetization of the reproduction layer is horizontal at a room temperature.
The second method is MAMMOS(Magnetic AMplifying Magneto-Optical System) which is applied to a case when a recording density is made very high, the small recorded domain in a recording layer is enlarged in a reproducing layer for making a reproduced signal larger. Here, a GdFeCo alloy which has the magnetic easy axis with the horizontal direction is used as the reproduction layer in the first method, and a GdFeCo alloy which has the magnetic easy axis with the vertical direction is used as the reproduction layer in the second method.
However, the magneto-optical disk with a reproduction layer of GdFeCo alloy has problems in which the magneto-optical disk can not be actually used in a blue region because, even though the magneto-optical disk has a Kerr rotation angle of 0.25.about.0.3.degree. in a long wavelengths of infrared.about.red region, the magneto-optical disk has a small Kerr rotation angle below 0.2.degree. in the short wavelengths of blue region. That is, despite a large reproduction signal should be provided at a short wavelengths of light for improving the recording density, the foregoing magneto-optical disk can not provide the larger reproduction signal due to the small Kerr rotation angle at a short wavelength. Moreover, the transcription from a small magnetic domain recorded in the recording layer to the reproduction layer is unclear due to the small size of recorded domains with the Gaussian temperature profile of the reproduction laser beam, which causes a problem of poor resolution.